Steel for nitrocarburizing and nitrocarburized component using the steel as material

ABSTRACT

According to the present invention, it is possible to obtain steel for nitrocarburizing having a predetermined chemical composition, a bainite area ratio exceeding 50% and excellent machinability by cutting before nitrocarburizing, and having strength and toughness equivalent to conventional steel, such as SCr420 carburized steel material, and excellent fatigue properties after nitrocarburizing.

TECHNICAL FIELD

This disclosure relates to steel for nitrocarburizing andnitrocarburized components using the steel as material. In particular,the disclosure relates to steel for nitrocarburizing that has excellentfatigue properties after nitrocarburizing and is suitable for use inautomobiles and construction equipment and to nitrocarburized componentsusing the steel as a material.

BACKGROUND

Since excellent fatigue properties are desired for machine structuralcomponents such as automobile gears, surface hardening is generallyperformed. Carburizing treatment, induction quench hardening andnitriding treatment are well-known forms of surface hardening.

With carburizing treatment, carbon is caused to infiltrate and diffuseinto a high-temperature austenite region, yielding a deep hardeningdepth. Carburizing treatment is thus useful to improve fatigue strength.

However, since heat treatment distortion occurs, it is difficult toapply carburizing treatment to components that, from the perspective ofnoise or the like, require high dimensional accuracy.

Induction quench hardening is a process of quenching a surface part byhigh frequency induction heating and, like carburizing treatment, causesdegradation of dimensional accuracy.

Nitriding treatment is a process to harden a surface by causing nitrogento infiltrate and diffuse into a high-temperature region at or below theAc₁ critical point. The treatment is long, taking 50 to 100 hours, andrequires removal of a brittle compound layer on the surface aftertreatment.

Therefore, nitrocarburizing treatment has been developed for nitridingat approximately the same treatment temperature as nitriding treatmentyet in a short time. In recent years, nitrocarburizing treatment hasbecome commonly used on machine structural components and the like.During nitrocarburizing treatment, nitrogen and carbon aresimultaneously caused to infiltrate and diffuse into a temperatureregion at 500° C. to 600° C. to harden the surface, making it possibleto reduce the treatment time to half or less that of conventionalnitriding treatment.

However, whereas it is possible to increase the core hardness by quenchhardening during carburizing treatment, nitrocarburizing treatment isperformed at a temperature at or below the critical point of steel, thuscausing the core hardness not to increase and yielding nitrocarburizedmaterial with poorer fatigue strength than carburized material.

To improve the fatigue strength of nitrocarburized material, quenchingand tempering are generally performed before nitrocarburizing toincrease the core hardness. The resulting fatigue properties, however,cannot be considered sufficient. Furthermore, this approach increasesmanufacturing costs and reduces mechanical workability.

To address these problems, it has been proposed to form steel with achemical composition including Ni, Al, Cr and Ti, to age-harden the coreduring nitrocarburizing by Ni—Al and Ni—Ti intermetallic compounds or byCu compounds, and to precipitation-harden nitrides and carbides such asCr, Al and Ti in a nitrided layer of the surface (JP 5-59488 A, JP7-138701 A).

JP 2002-69572 A discloses cogging steel that contains 0.5% to 2% of Cuby hot forging and then air cooling the steel to provide a ferrite-basedmicrostructure with solute Cu, precipitating the Cu duringnitrocarburizing treatment at 580° C. for 120 minutes and, furthermore,concurrently precipitation-hardening Ti, V and Nb carbonitrides to yielda steel that, after the nitrocarburizing treatment, has excellentbending fatigue properties. JP 2010-163671 A discloses steel fornitrocarburizing having dispersed therein Ti—Mo carbides and carbidesincluding at least one element selected from the group consisting of Nb,V and W.

While the nitrocarburizing steel recited in JP 5-59488 A and JP 7-138701A improves bending fatigue strength through precipitation-hardening ofCu and the like, the resulting workability cannot be consideredsufficient. By requiring the addition of a relatively large amount ofCu, Ti, V and Nb, the nitrocarburizing steel recited in JP 2002-69572 Ahas a high production cost. The steel for nitrocarburizing recited in JP2010-163671 A has the problem of high production cost due to theinclusion of a relatively large amount of Ti and Mo.

In view of the foregoing, it could be helpful to provide steel fornitrocarburizing and a nitrocarburized component using the steel asmaterial, the steel having a low hardness and excellent mechanicalworkability before nitrocarburizing while allowing for an increase incore hardness via nitrocarburizing treatment and allowing for relativelyinexpensive manufacture of nitrocarburized components with excellentfatigue properties.

SUMMARY

We intensely studied the effects of the microstructure and compositionof steel on the fatigue properties after nitrocarburizing of steel. As aresult, we discovered that with a steel material provided with aspecific amount of V and Nb in the steel composition and a bainite-basedmicrostructure before nitrocarburizing, excellent fatigue properties areobtained after nitrocarburizing by performing nitrocarburizing treatmenton the steel material while utilizing the rise in temperature toincrease the core hardness by age precipitating fine precipitates in thecore structure other than the nitrocarburized surface part.

We thus provide:

[1] A steel for nitrocarburizing comprising, in, mass %, C: 0.01% ormore and less than 0.10%, Si: 1.0% or less, Mn: 0.5% to 3.0%, Cr: 0.30%to 3.0%, Mo: 0.005% to 0.4%, V: 0.02% to 0.5%, Nb: 0.003% to 0.15%, Al:0.005% to 0.2%, S: 0.06% or less, P: 0.02% or less, B: 0.0003% to 0.01%,and the balance being Fe and incidental impurities, and including amicrostructure with a bainite area ratio exceeding 50% beforenitrocarburizing.

[2] The steel for nitrocarburizing according to [1], wherein afternitrocarburizing, precipitates including V and Nb are dispersed in abainite phase.

[3] A nitrocarburized component using the steel for nitrocarburizingaccording to [1] or [2] as material.

It is thus possible to obtain steel for nitrocarburizing, andnitrocarburized components using the steel as material, that hasexcellent machinability by cutting before nitrocarburizing, and thatafter nitrocarburizing has strength and toughness equivalent toconventional steel, such as SCr420 carburized steel material, andexcellent fatigue properties, thus proving extremely useful inindustrial terms.

BRIEF DESCRIPTION OF THE DRAWINGS

Our steels will be further described below with reference to theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram illustrating the manufacturing process tomanufacture a nitrocarburized component using steel fornitrocarburizing.

DETAILED DESCRIPTION

The microstructure, chemical composition and manufacturing conditions ofthe steel for nitrocarburizing will be described.

1. Microstructure

The microstructure before nitrocarburizing has a bainite area ratioexceeding 50%, and the microstructure after nitrocarburizing has V andNb precipitates dispersed in a bainite phase. When a matrix phase beforenitrocarburizing is a bainite-based microstructure with a bainite arearatio exceeding 50%, formation of V and Nb precipitates in the matrixphase is drastically inhibited compared to a ferrite-pearlitemicrostructure. As a result, formation of the V and Nb precipitatesbefore nitrocarburizing and consequent increased hardness of the steelcan be prevented, thereby improving workability of cutting generallyperformed before nitrocarburizing. Furthermore, applyingnitrocarburizing treatment to the steel causes the surface part to benitrided and simultaneously age precipitates the V and Nb precipitatesin the core bainite phase other than the nitrided surface part, therebyincreasing the core hardness. Both the fatigue properties and thestrength after nitrocarburizing therefore dramatically improve.

Note that the “microstructure with a bainite area ratio exceeding 50%”contemplated herein refers to the area ratio of the bainitemicrostructure (phase) exceeding 50% under cross-sectionalmicrostructure observation (microstructure observation with a 200×optical microscope). The area ratio of the bainite phase preferablyexceeds 60% and even more preferably exceeds 80%. Moreover, the V and Nbprecipitates in the bainite phase are preferably a dispersion of fineprecipitates having a grain size of less than 10 nm. Furthermore, forsufficient strengthening by precipitation, 500 or more of the V and Nbprecipitates with the grain size of less than 10 nm preferably exist per1 μm².

2. Chemical Composition

Reasons for the limitations of the chemical composition in the steel fornitrocarburizing will now be described. The fraction of each steelcomponent represents mass %.

C: 0.01% or More and Less Than 0.10%

Carbon (C) is added for bainite phase formation and to ensure strength.When the amount of C added is less than 0.01%, the amount of bainiteformed decreases, as does the amount of V and Nb precipitates, thusmaking it difficult to ensure strength. On the other hand, when 0.10% orgreater of C is added, the bainite phase becomes harder, therebyreducing the mechanical workability. Accordingly, the amount of C addedis 0.01% or more and less than 0.10%. C preferably 0.03% or more andless than 0.10%.

Si: 1.0% or Less

Silicon (Si) is added for its usefulness in deoxidizing and bainitephase formation. Adding an amount of Si exceeding 1.0%, however,deteriorates mechanical workability and cold-rolling workability due tosolid solution hardening of ferrite and bainite phases. Accordingly, theamount of Si added is 1.0% or less. The amount is preferably 0.5% orless and more preferably 0.3% or less. Note that for Si to contributeeffectively to deoxidation, the amount of Si added is preferably 0.01%or more.

Mn: 0.5% to 3.0%

Manganese (Mn) is added for its usefulness in bainite phase formationand in increasing strength. When the amount of Mn added is less than0.5%, the amount of bainite phase formed decreases, and V and Nbprecipitates are formed, causing the hardness before nitrocarburizing toincrease and the amount of V and Nb precipitates formed afternitrocarburizing treatment to decrease. In turn, this lowers thehardness after nitrocarburizing and makes it difficult to ensurestrength. On the other hand, adding an amount of Mn exceeding 3.0%deteriorates mechanical workability and cold-rolling workability.Accordingly, the amount of Mn added is 0.5% to 3.0%. The amount ispreferably 0.5% or more and 2.5% or less, and more preferably 0.6% ormore and 2.0% or less.

Cr: 0.30% to 3.0%

Chromium (Cr) is added for its usefulness in bainite phase formation.When the amount of Cr added is less than 0.30%, the amount of bainitephase formed decreases, and V and Nb precipitates are formed, causingthe hardness before nitrocarburizing to increase and the amount of V andNb precipitates formed after nitrocarburizing treatment to decrease. Inturn, this lowers the hardness after nitrocarburizing and makes itdifficult to ensure strength. On the other hand, adding an amount of Crexceeding 3.0% deteriorates mechanical workability and cold-rollingworkability. Accordingly, the amount of Cr added is 0.30% to 3.0%. Theamount is preferably 0.5% or more and 2.0% or less, and more preferably0.5% or more and 1.5% or less.

V: 0.02% to 0.5%

Vanadium (V) forms fine precipitates along with Nb due to the rise intemperature during nitrocarburizing and is therefore an importantelement to increase core hardness and improve strength. An added amountof V less than 0.02% does not satisfactorily achieve these effects. Onthe other hand, adding an amount of V exceeding 0.5% causes theprecipitates to coarsen. Accordingly, the amount of V added is 0.02% to0.5%. The amount is preferably 0.03% or more and 0.3% or less, and morepreferably 0.03% or more and 0.25% or less.

Nb: 0.003% to 0.15%

Niobium (Nb) forms fine precipitates along with V due to the rise intemperature during nitrocarburizing and is therefore an extremelyeffective element to increase core hardness and improve fatiguestrength. An added amount of Nb less than 0.003% does not satisfactorilyachieve these effects. On the other hand, adding an amount of Nbexceeding 0.15% causes the precipitates to coarsen. Accordingly, theamount of Nb added is 0.003% to 0.15%. The amount is preferably 0.02% ormore and 0.12% or less.

Mo: 0.005% to 0.4%

Molybdenum (Mo) causes fine V and Nb precipitates to form and iseffective in improving the strength of the nitrocarburized material. Mois therefore an important element. Mo is also useful for bainite phaseformation. To improve strength, 0.005% or more is added, but since Mo isan expensive element, adding more than 0.4% leads to increased componentcost. Accordingly, the amount of Mo added is 0.005% to 0.4%. The amountis preferably 0.01% to 0.3% and more preferably 0.04% to 0.2%.

Al: 0.005% to 0.2%

Aluminum (Al) is a useful element to improve surface hardness andeffective hardened case depth after nitrocarburizing and is thereforeintentionally added. Al also yields a finer microstructure by inhibitingthe growth of austenite grains during hot forging and is thus a usefulelement to improve toughness. Therefore, 0.005% or more is added. On theother hand, including over 0.2% does not increase this effect, butrather causes the disadvantage of higher component cost. Accordingly,the amount of Al added is 0.005% to 0.2%. The amount is preferably over0.020% and 0.1% or less, and more preferably over 0.020% and 0.040% orless.

S: 0.06% or Less

Sulfur (S) forms MnS in the steel and is a useful element to improve themachinability by cutting. Including over 0.06%, however, lessenstoughness. Accordingly, the amount of S added is 0.06% or less. Theamount is preferably 0.04% or less. Note that for S to achieve theeffect of improving machinability by cutting, the amount of S added ispreferably 0.002% or more.

P: 0.02% or Less

Phosphorus (P) exists in a segregated manner at austenite grainboundaries and lowers the grain boundary strength, thereby loweringstrength and toughness. Accordingly, the P content is preferably kept aslow as possible, but a content of up to 0.02% is tolerable. The Pcontent is therefore 0,02% or less. Note that setting the content of Pto less than 0.001% requires a high cost. Therefore, it suffices inindustrial terms to reduce the content of P to 0.001%.

B: 0.0003% to 0.01%

Boron (B) effectively promotes bainite phase formation. An added amountof B less than 0.0003% does not satisfactorily achieve this effect. Onthe other hand, adding over 0.01% does not increase this effect and onlyleads to higher component cost. Accordingly, the amount of B added is0.0003% to 0.01%. The amount is preferably 0.0010% or more and 0.01% orless.

Note that to achieve the effect of promoting bainite phase formation, itis preferable that B be present in the steel as a solute. When solute Nis present in the steel, however, the B in the steel is consumed byformation of BN. B does not contribute to improved quench hardenabilitywhen existing in the steel as BN. Accordingly, when solute N exists inthe steel, B is preferably added in an amount greater than that consumedby formation of BN, and the amounts of B (% B) and of N (% N) in thesteel preferably satisfy formula (1) below.

% B≧% N/14×10.8+0.0003.   (1)

In the steel for nitrocarburizing, after subjection to forging or whenimproving machinability by cutting of the nitrocarburized material, oneor more selected from the group of Pb≦0.2% and Bi≦0.02% may be added.Note that the desired effects achieved are not diminished regardless ofwhether these elements are added and regardless of their content.

Furthermore, in the steel for nitrocarburizing, the balance other thanthe above added elements consists of Fe and incidental impurities. Inparticular, however, Ti not only adversely affects strengthening byprecipitation of V and Nb, but also lowers the core hardness andtherefore is not to be included insofar as possible. The amount of Ti ispreferably less than 0.010% and more preferably less than 0.005%.

3. Manufacturing Conditions

FIG. 1 is a schematic diagram illustrating the manufacturing process ofmanufacturing a nitrocarburized component using steel fornitrocarburizing according to the present invention.

In FIG. 1, S1 indicates a manufacturing process of a steel bar as amaterial, S2 indicates a transportation process, and S3 indicates theprocess of finishing the product (nitrocarburized component).

Specifically, in the steel bar manufacturing process (S1), a steel ingotis hot rolled into a steel bar and shipped after quality inspection.After shipping, the steel bar is transported (S2), and during theprocess (S3) of finishing the product (nitrocarburized component), thesteel bar is cut to predetermined dimensions and subjected to hotforging or cold forging. After cutting the steel bar into apredetermined shape by drill boring, lathe turning or the like asnecessary, nitrocarburizing treatment is performed, yielding the finalproduct.

Alternatively, hot rolling material may be directly cut into apredetermined shape by lathe turning, drill boring or the like, withnitrocarburizing treatment then being performed to yield the finalproduct. In the case of hot forging, cold straightening may be performedafterwards. Coating treatment such as painting or plating, may also beapplied to the final product. Preferable manufacturing conditions willnow be described.

Rolling Heating Temperature

The rolling heating temperature is preferably 950° C. to 1250° C. Thisrange is adopted to cause carbides remaining after melting to be presentas a solute during hot rolling, so as not to diminish forgeability dueto formation of fine precipitates in the rolling material (the steel barwhich is the material for the hot forging component).

In other words, when the rolling heating temperature is less than 950°C., it becomes difficult for the carbides remaining after melting toform a solute. On the other hand, a temperature exceeding 1250° C.facilitates coarsening of the crystal grains, thus reducingforgeability. Accordingly, the rolling heating temperature is preferably950° C. to 1250° C.

Rolling Finishing Temperature

The rolling finishing temperature is preferably 800° C. or more. Thistemperature is adopted because at a rolling finishing temperature ofless than 800° C., a ferrite phase forms. Particularly when the nextprocess is nitrocarburizing after cold forging or cutting, such aferrite phase is disadvantageous to obtain a bainite phase with an arearatio exceeding 50% of the matrix phase after nitrocarburizing.Moreover, at a rolling finishing temperature of less than 800° C., therolling load increases, which degrades the out-of-roundness of therolling material. Accordingly, the rolling finishing temperature ispreferably 800° C. or more.

Cooling Rate

To prevent fine precipitates from forming before forging, therebyreducing forgeability, it is preferable to specify the cooling rateafter rolling. In the precipitation temperature range of fineprecipitates of 700° C. to 550° C., it is preferable to cool the steelbar faster than the critical cooling rate at which fine precipitates areproduced (0.5° C./s).

Nitrocarburizing Treatment (Precipitation Treatment)

The resulting steel bar is then used as material that is forged andshaped into components by cutting and the like. Nitrocarburizingtreatment is then performed. The temperature for nitrocarburizingtreatment is preferably 550° C. to 700° C. to yield fine precipitatesincluding V and Nb, and the treatment time is preferably 10 minutes ormore. This range is adopted because at less than 550° C., insufficientprecipitates are obtained, whereas over 700° C., the temperature entersthe austenite region, making nitrocarburizing difficult. A morepreferable range is 550° C. to 630° C. Furthermore, the treatment timeis 10 minutes or more to obtain a sufficient amount of V and Nbprecipitates.

Note that when hot forging is used, the hot forging is preferablyperformed with the heating temperature during hot forging at 950° C. to1250° C., with the forging finishing temperature at 800° C. or more andthe cooling rate after forging exceeding 0.5° C./s for the bainite phaseto exceed 50% in area ratio of the matrix phase after nitrocarburizingand in order to prevent formation of fine precipitates from thestandpoints of cold straightening and workability of cutting after hotforging.

EXAMPLES

Next, our steels are further described by examples.

Steel samples with the composition shown in Table 1 (steel samples No. 1to 17) were obtained by steelmaking in a 150 kg vacuum melting furnace,then rolling by heating at 1150° C., finishing at 970° C., andsubsequently cooling to room temperature at a cooling rate of 0.9° C./sto prepare steel bars with o 50 mm. No. 17 is a conventional material,JIS SCr420. Note that P was not intentionally added to any of the steelsamples in Table 1. Accordingly, the content of P in Table 1 indicatesthe amount mixed in as an incidental impurity. Furthermore, Ti was addedto steel samples No. 14 and No. 15 but not intentionally added to steelsamples No. 1 to 13 and No. 16 to 17 in Table 1. Accordingly, thecontent of Ti in steel samples No. 1 to 13 and No. 16 to 17 in Table 1indicates the amount mixed in as an incidental impurity.

These materials were then heated to 1200° C. and subsequently hot forgedat 1100° C. to a size of o 30 mm. The materials were cooled to roomtemperature at a cooling rate of 0.8° C./s, with a portion being cooledat 0.1° C./s for the sake of comparison.

TABLE 1 (mass %) Steel Sample No. C Si Mn P S Cr Mo V Nb Al Ti B NCategory 1 0.038 0.07 1.82 0.012 0.020 0.61 0.20 0.18 0.09 0.032 0.0010.0051 0.0056 Inventive Example 2 0.049 0.18 1.14 0.010 0.017 1.13 0.130.13 0.04 0.025 0.002 0.0074 0.0084 Inventive Example 3 0.077 0.24 0.730.015 0.020 1.42 0.07 0.29 0.12 0.024 0.002 0.0050 0.0055 InventiveExample 4 0.086 0.29 0.64 0.018 0.034 1.20 0.10 0.14 0.03 0.029 0.0030.0078 0.0090 Inventive Example 5 0.089 0.16 0.85 0.013 0.019 0.79 0.200.11 0.10 0.037 0.001 0.0068 0.0061 Inventive Example 6 0.050 0.25 1.350.019 0.031 1.01 0.05 0.14 0.06 0.025 0.004 0.0055 0.0055 InventiveExample 7 0.170 0.22 0.70 0.017 0.025 1.13 0.19 0.13 0.06 0.024 0.0020.0069 0.0077 Comparative Example 8 0.081 1.10 3.15 0.014 0.015 0.640.14 0.14 0.05 0.029 0.002 0.0055 0.0057 Comparative Example 9 0.0790.28 0.34 0.018 0.027 1.20 0.07 0.19 0.10 0.028 0.003 0.0053 0.0056Comparative Example 10 0.069 0.23 1.01 0.016 0.022 0.27 0.09 0.14 0.060.028 0.001 0.0057 0.0059 Comparative Example 11 0.048 0.08 1.04 0.0110.018 0.85  0.003 0.13 0.06 0.026 0.003 0.0059 0.0064 ComparativeExample 12 0.073 0.11 0.94 0.011 0.016 1.08 0.12 0.01  0.001 0.025 0.0030.0060 0.0061 Comparative Example 13 0.040 0.06 1.68 0.014 0.019 1.150.10 0.12  0.001 0.030 0.001 0.0049 0.0051 Comparative Example 14 0.0390.08 1.65 0.014 0.022 1.20 0.08 0.12 0.04 0.029 0.030 0.0048 0.0053Comparative Example 15 0.037 0.09 1.66 0.012 0.018 1.16 0.12 0.16 0.050.025 0.100 0.0045 0.0054 Comparative Example 16 0.065 0.15 1.13 0.0100.016 0.85 0.10 0.14 0.05 0.004 0.002 0.0058 0.0062 Comparative Example17 0.220 0.27 0.79 0.014 0.018 1.18  0.001  0.005  0.001 0.027 0.0040.0001 0.0105 Conventional Example

The microstructure of the above materials was observed, hardness wasmeasured, and machinability by cutting was tested. During microstructureobservation, a cross-section was observed under an optical microscope,and the core microstructure was identified. For samples in which abainite phase was present in the core, the area fraction of the bainitephase in the core was calculated. Machinability by cutting was assessedby a drill cutting test. Specifically, hot forging material was slicedto yield 20 mm thick pieces of test material in which through holes werebored in five locations per cross section using a JIS high-speed toolsteel SKH51 straight drill with a 6 mm, under the following conditions:feed rate, 0.15 mm/rev; revolution speed, 795 rpm. Machinability bycutting was assessed by the total number of holes before the drill couldno longer cut.

Hardness was measured by testing the hardness of the core using aVickers hardness tester, with a test force of 100 g.

For steel samples No. 1 to 16, gas nitrocarburizing treatment wasfurther applied to the hot forging material, and for steel sample No.17, gas carburizing treatment was applied to the hot forging material.The gas nitrocarburizing treatment was performed by heating to 570° C.to 620° C. and retaining for 3.5 h under an atmosphere ofNH₃:N₂:CO₂=50:45:5. The gas carburizing treatment was performed bycarburizing at 930° C. for 3 h, then oil quenching after retaining at850° C. for 40 minutes and, furthermore, tempering at 170° C. for 1 h.

The microstructure of these heat treatment materials was observed,hardness measured, precipitates observed, and impact properties andfatigue properties tested.

During microstructure observation, a cross-section was observed under anoptical microscope, and the core microstructure was identified. Forsamples in which a bainite phase was present in the core, the areafraction of the bainite phase was calculated.

To measure the hardness of the nitrocarburized material and thecarburized material, the core hardness and surface hardness weremeasured. The surface hardness was measured at a position 0.02 mm fromthe surface, and the effective hardened case depth was measured as thedepth from the surface at a hardness of HV 400. Samples for transmissionelectron microscopy observation were created from the cores of thenitrocarburized material and the carburized material by Twin-jetelectropolishing. Precipitates were observed in the resulting samplesusing a transmission electron microscope with an acceleration voltage of200 kV. Furthermore, the composition of the observed precipitates wascalculated with an energy-dispersive X-ray spectrometer (EDX).

The assessment of impact properties was made by performing a Charpyimpact test and calculating the impact value (J/cm²). Notched testpieces (R: 10 mm, depth: 2 mm) were used as test pieces. The notchedtest pieces were collected from the hot forging material, and afterperforming the above-described nitrocarburizing treatment or carburizingtreatment, the collected test pieces were used in the Charpy impacttest.

The assessment of fatigue properties was made by an Ono-type rotarybending fatigue test, and the fatigue limit was calculated. Notched testpieces (notch R: 1.0 mm; notch diameter: 8 mm; stress concentrationfactor: 1.8) were used as test pieces. The test pieces were collectedfrom the hot forging material and, after the above-describednitrocarburizing treatment or carburizing treatment, were used in thefatigue test.

Table 2 shows the test results. No. 1 to 6 are our examples, No. 7 to 17are comparative examples, and No. 18 is a conventional example providedby JIS SCr420 steel.

TABLE 2 Characteristics Before Characteristics After Cooling Rate AfterNitrocarburizing Nitrocarburizing Treatment Steel Heat Treatment CoreCore Bainite Drill Nitrocarburizing Surface Sample Corresponding to HotHardness Structure Phase Area Hole Treatment Hardness No. No. Forging (°C./s) HV (1) Ratio (%) Count Temperature (° C.) HV 1  1 0.8 240 B-based98 496 605 787 2  2 0.8 244 B-based 92 487 570 795 3  3 0.8 264 B-based96 441 620 805 4  4 0.8 268 B-based 97 431 590 796 5  5 0.8 266 B-based92 436 590 784 6  6 0.8 240 B-based 90 495 590 790 7  2 0.1 228 F + P  0524 590 787 8  7 0.8 290 B-based 94 200 590 799 9  8 0.8 323 M + B 38 89590 786 10  9 0.8 290 F + P + B 12 193 590 801 11 10 0.8 284 F + P + B15 198 590 837 12 11 0.8 213 B-based 65 577 590 787 13 12 0.8 252B-based 96 470 590 795 14 13 0.8 242 B-based 97 491 590 790 15 14 0.8241 B-based 97 499 590 788 16 15 0.8 244 B-based 98 492 590 795 17 160.8 249 B-based 96 479 590 724 18 17 0.8 248 F + P + B 85 449 930° C. ×3 h 730 carburizing, 850° C. × 40 m retaining then oil quenching, 170°C. × 1 h tempering Characteristics After Nitrocarburizing TreatmentEffective Core Core Bainite Impact Fatigue Hardened Case HardnessStructure Phase Area Value Strength No. Depth (mm) HV (1) Ratio (%)(J/cm²) (MPa) Category 1 0.15 295 B-based 98 12 512 Inventive Example 20.17 277 B-based 92 12 476 Inventive Example 3 0.19 324 B-based 96 11577 Inventive Example 4 0.17 300 B-based 97 12 525 Inventive Example 50.15 294 B-based 92 13 509 Inventive Example 6 0.16 277 B-based 90 12474 Inventive Example 7 0.15 226 F + P 0 13 347 Comparative Example 80.17 319 B-based 94 11 566 Comparative Example 9 0.15 353 M + B 38 12640 Comparative Example 10 0.17 298 F + P + B 12 13 527 ComparativeExample 11 0.17 295 F + P + B 15 13 514 Comparative Example 12 0.18 232B-based 65 12 373 Comparative Example 13 0.16 250 B-based 96 12 416Comparative Example 14 0.17 253 B-based 97 13 423 Comparative Example 150.17 251 B-based 97 3 418 Comparative Example 16 0.19 260 B-based 98 2450 Comparative Example 17 0.12 279 B-based 96 9 395 Comparative Example18 1.05 360 Tempered M + B 50 15 470 Conventional Example (1) F:Ferrite, P: Pearlite, B: Bainite, M: Martensite

As is clear from Table 2, nitrocarburized materials No. 1 to 6 havebetter fatigue strength than the material resulting from carburizing,quenching, and tempering the conventional example (No. 18). As forworkability of drill cutting, the material before nitrocarburizingtreatment in No. 1 to 6 (hot forging material) has a level equivalent toor greater than the conventional material in practical terms.Furthermore, the results of transmission electron microscopy observationand of testing the precipitate composition by EDX confirm that thenitrocarburized materials No. 1 to 6 contain 500 or more fineprecipitates, including V and Nb, with a grain size of less than 10 nmdispersed per 1 μm² in the bainite phase. Based on these results, it canbe concluded that our nitrocarburized material exhibits a high fatiguestrength due to strengthening by precipitation based on the above fineprecipitates.

By contrast, comparative examples No. 7 to 17 have a chemicalcomposition or a resulting microstructure that are outside of our scopeand thus have worse fatigue strength or drill workability.

In particular, No. 7 has low fatigue strength as compared to ourexamples due to the slow cooling rate after hot forging. For No. 7, theresults of transmission electron microscopy observation showed nodispersion of fine precipitates with a grain size of less than 10 nm,whereas course precipitates with a grain size greatly exceeding 10 nmwere observed. Based on these results, the coarseness of such resultingprecipitates can be considered the cause of the reduction in fatiguestrength. In other words, we believe that if the cooling rate after hotforging is slow and the desired bainite phase is not obtained, courseprecipitates are formed before nitrocarburizing. The amount of fineprecipitates that form after nitrocarburizing treatment then decreases,resulting in insufficient strengthening by precipitation.

No. 8 includes a high amount of C, outside of our range. The hardness ofthe bainite phase therefore increases, reducing drill workability.

No. 9 includes high amounts of Si and Mn, outside of our range. Thehardness of the hot forging material is therefore high, reducing thedrill workability to approximately ⅕ that of conventional material.

No. 10 includes a low amount of Mn, outside of our range. Aferrite-pearlite microstructure thus forms before nitrocarburizing(after hot forging), lowering the area ratio of the bainite phase andforming V and Nb precipitates in the microstructure. The hardness beforenitrocarburizing thus increases, reducing the drill workability.

No. 11 includes a low amount of Cr, outside of our range. Aferrite-pearlite microstructure thus forms before nitrocarburizing(after hot forging), lowering the area ratio of the bainite phase andforming V and Nb precipitates in the microstructure. The hardness beforenitrocarburizing thus increases, reducing the drill workability.

No. 12 includes a low amount of Mo, outside of our range. Therefore, fewfine precipitates exist after the nitrocarburizing treatment, and theresulting core hardness is insufficient. The fatigue strength istherefore lower than the conventional example.

No. 13 includes low amounts of V and Nb, outside of our range.Therefore, few fine precipitates exist after the nitrocarburizingtreatment, and the resulting core hardness is insufficient. The fatiguestrength is therefore lower than the conventional material.

No. 14 includes a low amount of Nb, outside of our range. Therefore, fewfine precipitates exist after the nitrocarburizing treatment, and theresulting core hardness is insufficient. The fatigue strength istherefore lower than the conventional material.

Ti was added to No. 15 and No. 16, thus yielding few precipitatesincluding V and Nb after the nitrocarburizing treatment. The resultingcore hardness is therefore insufficient, and the fatigue strength islower than the conventional material. Furthermore, the impact value islow.

No. 17 includes a low amount of Al, outside of our range. The surfacehardness after the nitrocarburizing treatment and the effective hardenedcase depth are therefore insufficient, resulting in a lower fatiguestrength than the conventional material.

1.-3. (canceled)
 4. A steel for nitrocarburizing comprising, in mass %,C: 0.01% or more and less than 0.10%, Si: 1.0% or less, Mn: 0.5% to3.0%, Cr: 0.30% to 3.0%, Mo: 0.005% to 0.4%, V: 0.02% to 0.5%, Nb:0.003% to 0.15%, Al: 0.005% to 0.2%, S: 0.06% or less, P: 0.02% or less,B: 0.0003% to 0.01%, and the balance being Fe and incidental impurities,and including a microstructure with a bainite area ratio exceeding 50%before nitrocarburizing.
 5. The steel according to claim 4, whereinafter nitrocarburizing, precipitates including V and Nb are dispersed ina bainite phase.
 6. A nitrocarburized component comprising a steelproduced by nitrocarburizing the steel according to claim
 4. 7. Anitrocarburized component comprising a steel produced bynitrocarburizing the steel according to claim 5.